Function requires_visibility_check 

fn requires_visibility_check(target: ChannelId, source: ChannelId) -> bool

Returns true when the link’s visibility must be validated against the request’s source channel.

A link pointing back into the same channel the request came from is always safe to expand: the reply lands in that very channel, so it cannot expose anything its readers cannot already see. Such links need no visibility checks, while links to any other channel do.